Method for passivating stainless steel product and method for producing stainless steel separator for fuel cell

ABSTRACT

By a press-formed starting member ( 11 ) being immersed in a process liquid ( 12 ) at 40 to 60° C. and of pH 9 to 12, a passive film ( 26 ) is formed at the surface of the press-formed starting member. There is no solving out of metal ions from the press-formed starting member, and the passivation treatment can be carried out by just immersion in a single solution, so that costs including waste processing can be reduced. In another aspect, degreasing, cleaning and passivation treatment are carried out by spraying the press-formed starting member. By the use of spraying, the degreasing, cleaning and passivation treatment can be made rapid, and the amounts of process liquid needed for degreasing, cleaning and passivation treatment can be reduced.

TECHNICAL FIELD

This invention relates to a passivation treatment method for a stainlesssteel member for use as a separator in a fuel cell and to a method ofmanufacturing a stainless steel separator.

BACKGROUND ART

Because solid polymer electrolyte membrane type fuel cells are of aconstruction such that a desired output is obtained by a number ofindividual fuel cell cells being stacked together, as separators fordividing the individual fuel cells, metal materials having superiorstrength with respect to pressurization on stacking and compactnessafter stacking compared to resin materials are being seen as effective.In particular, because it forms a passive film having a high resistanceto corrosion in acidic atmospheres around the electrode parts of cells,the use of stainless steel members is being studied.

As passivation treatment methods for forming this kind of passive filmon a stainless steel member, those in which an acidic solution is usedas a process liquid (for example (1) Japanese Laid-Open Patent No.61-270396, (2) Japanese Laid-Open Patent No. 9-184096 and (3) JapaneseLaid-Open Patent No. 2000-323152), and those in which a neutral orweakly acidic solution is used as a process liquid ((4) JapaneseLaid-Open Patent Publication No. 10-280163) are known.

In the above-mentioned publication (1), a passivation treatment methodfor passivation-treating a stainless steel member with for exampledilute nitric acid is set forth.

In the above-mentioned publication (2), a surface treatment method forelectrolytically treating stainless steel with an aqueous solution ofnitric acid and chromic acid is set forth.

In the above-mentioned publication (3), a separator manufacturing methodis set forth in which stainless steel is acid-washed with a mixture ofnitric acid and hydrofluoric acid and then a passive film is formed withnitric acid.

In the above-mentioned publication (4), a passivation treatment methodis set forth in which a stainless steel sheet is coated with a solutionin liquid film form containing a neutral salt electrolyte and hydrogenperoxide.

In the above-mentioned publication (1), publication (2) and publication(3), in each case, because an acid is used for the passivationtreatment, metal ions solve out into the process liquid. For example, inthe case of nitric acid, Ni²⁺ and Cr⁶⁺ solve out. Consequently, a greatdeal of cost is entailed in processing waste liquid including metal ionsafter the passivation treatment is finished.

And in the above-mentioned publication (4), because the process liquidis applied in a liquid film state, that is, applied for example using abrush or the like, by spraying using an atomizer, or by repeated dippingand exposure to the atmosphere, the number of steps is large,productivity is low, and as a result cost is high.

Also, as stainless steel treatment methods of this kind, those in whichdegreasing and passivation treatment are carried out (for example (5)Japanese Laid-Open Patent Publication No. 10-503241), those in whichdegreasing and etching are carried out (for example (6) JapaneseLaid-Open Patent Publication No. 9-291400), those in which degreasingand polishing are carried out (for example (7) Japanese Laid-Open PatentPublication No. 2000-282276), and those in which acid cleaning arecarried out (for example (8) Japanese Laid-Open Patent Publication No.2001-214286) are known.

The above-mentioned publication (5) provides an alkali-based compositionfor cleaning and passivating the surface of a stainless steel sheet, anda stainless steel surface cleaning liquid is made up of an alkalinecomponent other than alkaline salts such as carbonates+a chelatingagent+water+a surfactant.

In the above-mentioned publication (6), electrolysis treatment iscarried out with an aqueous solution of pH 10 to 12.5 including sodiumhydroxide and sulfuric acid as an electrolyte and thereby rolling oilattached to the surface after cold-rolling of the stainless steel isremoved and the concentration of Cr component in the passive film formedon the stainless steel surface is made low and the etchability of thematerial is improved.

In the above-mentioned publication (7), an alkaline solution is blown atthe surface of a cold-rolled stainless steel sheet while it is brushed,whereby smudges (dirt) existing on the surface of the steel sheet afterthe cold-rolling are removed, and then polishing is carried out, wherebya stainless steel polished product having a surface nature with nopattern or pit flaws is manufactured.

In the above-mentioned publication (8), the stainless steel isacid-washed with an acidic aqueous solution to expose at least one ofcarbide metal inclusions and boride metal inclusions having conductivityat its surface, and then a neutralizing treatment is carried out bymeans of an alkaline solution of pH 7 or greater to prevent any increasein electrical contact resistance, and then rinsing with water and dryingare further carried out.

The acid cleaning is carried out by immersing the stainless steel in theacidic aqueous solution or showering the acidic aqueous solution ontothe stainless steel surface.

Because in the above-mentioned publication (5) the stainless steel sheetis immersed in an alkali-based solution and in the above-mentionedpublication (6) the cold-rolled stainless steel is immersed in anaqueous solution including sodium hydroxide and sulfuric acid, forexample if cleaning is carried out by an overflow immersion method inwhich the process liquid is made to overflow from the process tankduring immersion, the amount of process liquid used becomes large, and,because with the immersion method the cleaning is carried out gradually,the process time becomes long as well. Also, if the amount of greaseattached to the stainless steel material is large, grease may remainthere after the cleaning, and certain cleaning sometimes cannot becarried out.

In the publication (8), because different acid and alkali solutions areused, processing of waste liquids must be carried out separately, andcosts mount up.

Also, in the publication (7), to effect brushing while an alkalinesolution is blown at the stainless steel sheet, a drive source forproducing a driving force for brushing is necessary. If processing canbe carried out with simple equipment, the cost of the equipment can bereduced.

DISCLOSURE OF THE INVENTION

It is an object of the present invention, with respect to a passivationtreatment method for a stainless steel member, to make the wasteprocessing of process liquids easy and furthermore to make the number ofprocess steps low and reduce the cost incurred in passivation treatment,and, with respect to a manufacturing method of a stainless steelseparator for a fuel cell, to shorten the time required for cleaning astainless steel member serving as the material of a separator and reducethe amount of process liquid needed for cleaning, to suppress wasteliquid processing costs and achieve simplification of equipment.

The invention provides a stainless steel member passivation treatmentmethod in which a passive film is formed at the surface of a stainlesssteel member by the stainless steel member being dipped in an alkalinesolution at 40 to 60° C. and of pH 9 to 12.

By a stainless steel member being passivation-treated with an alkalinesolution, because there is no solving out of metal ions from thestainless steel member, compared to passivation-treating with acid, thecost incurred in waste liquid processing can be reduced.

Passivation treatment can be carried out with just dipping in a singlesolution, and the number of process steps is low and it is possible toachieve still more cost reduction.

Preferably, a pH buffer is added to the alkaline solution, or thealkaline solution itself is provided with a pH-buffer action, and by airbubbling being carried out into this alkaline solution, the formation ofhydroxides to constitute a passive film is promoted by an increase inthe amount of dissolved oxygen in the alkaline solution, and alsofalling of the pH is suppressed by carbon dioxide dissolving in thealkaline solution.

When air bubbling is carried out, OH⁻ increases as a result of theamount of dissolved oxygen in the alkaline solution increasing, andbecause the increased OH⁻ bonds with metal ions the formation ofhydroxides can be promoted.

Because the air bubbling also causes carbon dioxide to dissolve in thealkaline solution as well as oxygen, due to this carbon dioxide the H⁺in the alkaline solution increases and causes the pH to fall, but byadding a pH buffer to the alkaline solution in advance, or providing thealkaline solution itself with a pH buffer effect, it is possible tosuppress falling of the pH with the pH buffer or the alkaline solutionitself.

Preferably, the stainless steel member having finished the dipping stepis dried by being held at 100 to 200° C.

By thermal drying at 100 to 200° C., the passive film formed by thealkaline solution can be made more stable and resistance to corrosioncan be raised further.

Also, the stainless steel member is suitable for being made into a fuelcell separator.

When a fuel cell is generating electricity, the atmospheres around theelectrodes become acidic, but at the surface of a separator obtained bydipping in an alkaline solution of pH 9 to 12 and then thermal drying at100 to 200° C., as with nitric acid passivation treatment methods ofrelated art, because a passive film having Fe, Cr and Ni hydroxide andoxide components is formed, corrosion by oxygen can be suppressed, andstable electricity generation can be maintained over a long period.

In another aspect, the invention provides a manufacturing method of astainless steel separator for a fuel cell, which is made up of: a stepof applying a lubricant to a stainless steel member and press-forminggas flow passages and cooling water flow passages in it; a step ofremoving lubricant adhered to the stainless steel member by spraying thepress-formed stainless steel member with an alkaline solution forcleaning; a step of removing alkaline solution for cleaning adhered tothe stainless steel member by spraying washing water onto the stainlesssteel member; a step of removing washing water remaining on thestainless steel member by spraying ion-exchange water onto the stainlesssteel member; a step of spraying an alkaline solution for passivationtreatment onto the stainless steel member to passivation-treat thestainless steel member; a step of removing alkaline solution forpassivation treatment adhered to the stainless steel member by sprayingion-exchange water onto the stainless steel member; and a step ofthermally drying the stainless steel member. By the step of removinglubricant adhered to the stainless steel member by spraying an alkalinesolution for cleaning onto the stainless steel member being provided,the lubricant-removing effect can be raised by the spraying, andcompared to a related art removal step based on dipping the removal timecan be shortened and the amount of alkaline solution for cleaning neededfor removal can be reduced.

By the cleaning with washing water and ion-exchange water and thepassivation treatment being carried out by spraying, compared to arelated art dipping method it is possible to shorten the time needed forthese washes and passivation treatment further, and it is possible toreduce the amounts of washing water and ion-exchange water required forthe washing.

Also, because the solution for cleaning and the solution for passivationtreatment are both made alkaline, the respective waste liquids can beprocessed simultaneously, and costs can be kept down.

Also, because a drive source necessary for brushing or the like of thekind in related art is unnecessary, simplification of equipment can beachieved and the cost of equipment can be reduced.

Preferably, the alkaline solution for passivation treatment is made asolution of pH 9 to 12 brought to 40 to 60° C.

Because the stainless steel member is passivation-treated with analkaline solution for passivation treatment, there is no solving out ofmetal ions from the stainless steel member, and compared to passivationtreatment with acid the cost of waste liquid processing can be reduced,and passivation treatment can be carried out with just spraying with asingle solution, the number of process steps is small and further costreduction can be achieved.

Preferably, the alkaline solution for passivation treatment is asolution with a pH buffer added.

By the spray of alkaline solution for passivation treatment, falling ofthe pH caused by carbon dioxide dissolving in the alkaline solution forpassivation treatment can be suppressed by the pH buffer, and a passivefilm can be formed efficiently and stably.

Also, preferably, the thermal drying is carried out at 100 to 200° C.

By thermal drying at 100 to 200° C., the passive film formed with thealkaline solution can be made more stable, and resistance to corrosioncan be raised further.

Also, preferably, the alkaline solution for cleaning is a solution madeby adding a surfactant to a basic salt.

Because the alkaline solution for cleaning is a solution made by addinga surfactant to a basic salt, foaming does not readily occur, andproblems of draining and the like caused by foaming can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, FIG. 1B and FIG. 1C are explanatory views showing a stainlesssteel separator passivation treatment method according to the invention,FIG. 1A being a process view, and FIG. 1B and FIG. 1C being sectionalviews of separators in different steps.

FIG. 2 is a first graph showing results of a corrosion test on samplesincluding stainless steel members on which a passivation treatmentmethod according to the invention has been carried out.

FIG. 3 is a second graph showing results of a corrosion test on samplesincluding stainless steel members on which a passivation treatmentmethod according to the invention has been carried out.

FIG. 4 is a third graph showing results of a corrosion test on samplesincluding a stainless steel member on which a passivation treatmentmethod according to the invention has been carried out.

FIG. 5 is a fourth graph showing results of a corrosion test onstainless steel members confirming an effect of air bubbling and aneffect of thermal drying of a passivation treatment method according tothe invention.

FIG. 6 is a fifth graph showing results of a corrosion test on samplesincluding stainless steel members on which a passivation treatmentmethod according to the invention has been carried out.

FIG. 7 is a sixth graph showing an action of bubbling and a pH buffer ofa passivation treatment method according to the invention.

FIG. 8A, FIG. 8B and FIG. 8C are explanatory views showing a stainlesssteel separator manufacturing method according to the invention, FIG. 8Abeing a process view and FIG. 8B and FIG. 8C being sectional views of aseparator starting member in different steps.

FIG. 9 is an explanatory view comparing stainless steel separatormanufacturing methods.

FIG. 10A and FIG. 10B are explanatory views showing a first experimentexample for confirming an effect of spray cleaning in a stainless steelseparator manufacturing method according to the invention, FIG. 10Abeing a flow chart of sample preparation and effect confirmation andFIG. 10B a graph for comparison.

FIG. 11A and FIG. 11B are explanatory views showing a second experimentexample for confirming a cleaning time of spray cleaning in a stainlesssteel separator manufacturing method according to the invention, FIG.11A being a flow chart of sample preparation and effect confirmation andFIG. 11B a graph for comparison.

FIG. 12A and FIG. 12B are explanatory views showing a third experimentexample for confirming a rinsing time of spray rinsing in a stainlesssteel separator manufacturing method according to the invention, FIG.12A being a flow chart of sample preparation and effect confirmation andFIG. 12B a graph for comparison.

FIG. 13 is a graph showing a fourth experiment example for confirming apassivation treatment effect of spraying in a stainless steel separatormanufacturing method according to the invention.

FIG. 14 is a graph showing a fifth experiment example for confirming athrough resistance of a separator passivation-treated by spraying in astainless steel separator manufacturing method according to theinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

A separator passivation treatment method is illustrated in order in FIG.1A. STxx denotes a step number (and similarly hereinafter).

ST01: Multiple press-formed starting members 11 made by pressingstainless steel members are prepared.

The press-forming is carried out to form grooves for supplying fuel gasand oxidant gas to a fuel cell and draining produced water when theseparator finished in a final step is assembled to the fuel cell.

ST02: The multiple press-formed starting members 11 are immersed in aprocess tank 13 filled with a process liquid 12 (pH 9 to 12,concentration 0.00004 to 0.08 wt %) made by dissolving NaOH in distilledwater (pure water). The temperature of the process liquid 12 is 40 to60° C., the immersion time is 10 minutes, and air bubbling was carriedout. The amount of air bubbling is 1000 to 2000 cm³/min (this is thesame hereinafter).

Air bubbling means blowing air into the process liquid 12 to increasethe amount of dissolved oxygen in the process liquid 12 and therebypromote hydroxide formation. The mechanism of this promotion ofhydroxide formation by air bubbling is presumed to be as follows.

In the process liquid 12, the following reactions proceed.M=M^(n+) +ne ⁻  (a)1/2O₂+H₂O+2^(e) ⁻=2OH⁻  (b)M^(n+) +nOH⁻=M(OH)n  (c)

In equation (a) the metal M ionizes and, as a result of the airbubbling, OH⁻ is produced from oxygen, as shown by equation (b). As aresult, the OH⁻ in the process liquid 12 increases, and the productionof hydroxides from metal ions and the increased OH⁻ is promoted, asshown by equation (c).

And in the process liquid 12, as a result of the air bubbling thereactions shown below also proceed.CO₂+H₂O→HCO₃ ⁻+H⁺  (d)HCO₃ ⁻→CO₃ ²⁻+H⁺  (e)

As shown in expression (d), H⁺ is produced from carbon dioxide andwater, and as shown in expression (e), H⁺ is also produced from HCO₃ ⁻,so that H⁺ increases and consequently the pH of the process liquid 12falls.

However, because a pH buffer, for example Na₂CO₃ (CO₃ ²⁻ becomes aconjugate base) has been added to the process liquid 12 in advance,falling of the pH of the process liquid 12 can be suppressed.

ST03: The film-formed members 17 on which a passive film has been formedin ST02 are rinsed by being dipped in a water tank 16 filled withdistilled water (pure water) 14.

ST04: The rinsed film-formed members 17 are dried by heating in athermal dryer 18. The heating temperature is 100 to 200° C., and thethermal drying time is 10 minutes.

After the film-formed members 17 are dried, the separators are finished.

In FIG. 1B, a press-formed starting member 11 consists of a basematerial 22 and an altered layer 23 formed at the surface of this basematerial 22. 24 is conductors included in the base material 22.

The altered layer 23 is formed by the step of rolling the stainlesssteel member before the press-forming, and is made up of oxides andmetal inclusions included in the stainless steel sheet and broken upinto grains of small diameter.

FIG. 1C shows a passive film 26 having been formed at the surface of thebase material 22 of the film-formed member 17 by the passivationtreatment with the process liquid 12 (see FIG. 1A).

FIG. 2 shows data obtained by measuring the corrosion current density oftest pieces (which correspond to separators and thus have undergone thedrying by heating of ST04) having had passive films formed on them withthe pH of the aqueous NaOH in the passivation treatment step of ST02 inthe process shown in FIG. 1A made different in each case. If thecorrosion current density is small, it means that the member does notcorrode readily. The vertical axis of the graph shows corrosion currentdensity (in units of μA/cm²), and the horizontal axis shows pH of theaqueous NaOH. The dashed line is the corrosion current density (3.6μA/cm²) of a test piece of related art on which a passive film wasformed with nitric acid as the process liquid (this is the samehereinafter).

The corrosion test conditions are shown below.

Corrosion Test Conditions

-   -   test solution: aqueous sulfuric acid (pH 3, concentration        0.005%, temperature 90° C.)    -   test piece potential: constant 638.8 mV (set with reference to a        saturated calomel electrode (SCE)), hereinafter referred to as        “638.8 mV vs. SCE”    -   test method: measure corrosion current density after holding the        above test piece potential for 30 minutes

These test conditions are common to all the corrosion tests shown below.

Looking at the variation of the corrosion current density with the pH ofthe NaOH, when the pH of the aqueous NaOH is 7 or 8 the corrosioncurrent density exceeds 4 μA/cm², when the pH of the aqueous NaOH is 9to 12 the corrosion current density is approximately the same value aswhen the treatment was carried out with nitric acid, and when the pH ofthe aqueous NaOH is 13 the corrosion current density is greater thanwhen the treatment was carried out with nitric acid.

Thus it is desirable that the pH of the aqueous NaOH be 9 to 12.

FIG. 3 shows data obtained by measuring the corrosion current density oftest pieces having had passive films formed on them with the heatingtemperature of the thermal drying step of ST04 in the process shown inFIG. 1A made different in each case. At all of the heating temperaturesthe heating time is 10 minutes. The vertical axis of the graph showscorrosion current density (in units of μA/cm²), and the horizontal axisshows heating temperature (in units of ° C.). The corrosion currentdensity, shown with a dashed line, of the test piece of related art onwhich a passive film was formed with nitric acid as the process liquid,is 3.6 μA/cm² below about 210° C. and rises steeply above thistemperature.

With respect to one made with nitric acid as the process liquid, thecorrosion current density of the test piece made using the passivationtreatment of the embodiment shown in FIG. 1A is larger below 100° C.,about the same at 100 to 200° C., and larger at above 200° C.

Thus it is desirable for the heating temperature to be 100 to 200°.

FIG. 4 shows data obtained by measuring the corrosion current density offour test pieces having had passive films formed on them on the basis ofthe process shown in FIG. 1A. The vertical axis is corrosion currentdensity (in units of μA/cm²). The samples are as follows.

Sample

-   -   Sample A: passivation treatment and thermal drying not carried        out    -   Sample B: passivation treatment not carried out, thermal drying        carried out at 100° C. for 10 minutes    -   Sample C: immersion in aqueous NaOH of pH 10, 50° C. for 10        minutes carried out as a passivation treatment, and drying not        carried out    -   Sample D: immersion in aqueous NaOH of pH 10, 50° C. for 10        minutes carried out as a passivation treatment, and drying        carried out at 100° C. for 10 minutes

When Sample A and Sample B are compared, the corrosion current densityof Sample B is smaller. That is, the difference between these corrosioncurrent densities is an effect of the drying by heating (100° C., 10minutes).

When Sample A and Sample C are compared, the corrosion current densityof Sample C is smaller. That is, the difference between these corrosioncurrent densities is an effect of the passivation treatment (pH 10, 50°C., 10 minutes).

In Sample D, the corrosion current density is still smaller than inSample B and Sample C, and has about the same value as when thetreatment was carried out with nitric acid.

An immersion test and a corrosion resistance test shown in the tablebelow were carried out. TABLE 1 PASSIVATION TREATMENT CONDITIONSCORROSION PROCESS LIQUID IMMER- THERMAL DRYING IMMERSION TEST RESISTANCETEST CONCEN- TEMPER- SION AIR TEMPER- OCCURRENCE OF CORROSION TRATIONATURE TIME BUB- ATURE TIME RUST CURRENT DENSITY SAMPLE CONTENT pH wt % °C. Min. BLING ° C. min. Yes/No μA/cm² Embodiment Aqueous 10 0.0004 50 10Yes 110 10 No 0.1 1 NaOH Comparison Aqueous 10 0.0004 50 10 Yes — — —0.14 Example 1 NaOH Comparison Aqueous — 50 50 10 — Room — No 0.15Example 2 Nitric acid temperature

The immersion test was one in which a test piece having had a passivefilm formed on it in the process of FIG. 1A was taken as Embodiment 1,besides this a Comparison Example 2 was prepared, and each was immersedin acid for a long period and then inspected for rust, and was carriedout with the following conditions.

Immersion Test Conditions

-   -   test solution: aqueous sulfuric acid at pH 3, 90° C.    -   immersion time: 2200 hours continuous

The corrosion resistance test was one in which a test piece having had apassive film formed on it in the process of FIG. 1A (Embodiment 1) and aComparison Example 1 were prepared, and each was immersed in acid for along period and had a predetermined potential applied to it and after apredetermined time had its corrosion current density measured, and wascarried out with the following conditions.

Corrosion Resistance Test Conditions

-   -   test solution: aqueous sulfuric acid at pH 3, 90° C.    -   test piece potential: 638.8 mV vs. SCE    -   potential application time: 500 hours continuous

For Embodiment 1, as the passivation treatment conditions, the processliquid was aqueous NaOH of pH 10, concentration 0.0004 wt %, temperature50° C., the immersion time was 10 minutes, air bubbling was carried out,and the temperature of the drying by heating was 110° C. and the timewas 10 min.

The result was that there was no occurrence of rust in the immersiontest, and the corrosion current density in the corrosion resistance testwas 0.1 μA/cm².

For Comparison Example 1, as the passivation treatment conditions, theprocess liquid was aqueous NaOH of pH 10, concentration 0.0004 wt %,temperature 50° C., the immersion time was 10 minutes, air bubbling wascarried out, and drying by heating was not carried out.

The result was that the corrosion current density in the corrosionresistance test was 0.14 μA/cm².

For Comparison Example 2, as the passivation treatment conditions, theprocess liquid was aqueous nitric acid of concentration 50 wt %,temperature 50° C., the immersion time was 10 min, air bubbling was notcarried out, and drying was carried out at room temperature.

The result was that there was no occurrence of rust in the immersiontest, and the corrosion current density in the corrosion resistance testwas 0.15 μA/cm².

Thus, the passivation treatment conditions of the present invention(Embodiment 1) can form a passive film having the same resistance tocorrosion as the conditions of passivation treatment based on nitricacid carried out in related art (Comparison Example 2).

FIG. 5 shows data obtained by preparing four test pieces having hadpassive films formed on them on the basis of the process shown in FIG.1A and one test piece having had no passive film formed on it, andmeasuring their corrosion current densities. The vertical axis iscorrosion current density (in units of μA/cm²). The samples are asfollows.

Samples

-   -   Sample E: air bubbling and thermal drying (temperature 110° C.,        time 10 min (this temperature and time are the same in the other        samples in this test also)) carried out    -   Sample F: air bubbling not carried out, thermal drying carried        out    -   Sample G: air bubbling carried out, thermal drying not carried        out    -   Sample H: neither air bubbling nor thermal drying carried out    -   Sample J: unprocessed, i.e. not immersed in alkaline process        liquid and no passive film formed

Comparing Sample E and Sample F, when thermal drying was carried out,the corrosion current density of Sample E, on which air bubbling wascarried out, is smaller.

And comparing Sample G and Sample H, thermal drying was not carried outon either sample, and in the case of Sample G, with respect to Sample H,as a result of the air bubbling being carried out the amount of oxygendissolved in the alkaline process liquid approximately doubled from 3.8to 4.4 mg to 7.6 to 8.0 mg per 1000 cm³, and the corrosion currentdensity of Sample G was smaller.

The difference between the corrosion current densities of Sample E andSample F above and the difference between the corrosion currentdensities of Sample G and Sample H above are effects of air bubbling,and in Sample E the corrosion current density is substantially the sameas when treatment is carried out with nitric acid.

From a comparison of Sample E and Sample G, when air bubbling wascarried out, the corrosion current density of Sample E, where thermaldrying was carried out, is smaller, and thus the effect of thermaldrying is large. And from a comparison of Sample F and Sample H, whenair bubbling was not carried out, the corrosion current density ofSample F is smaller, and again the effect of thermal drying is large.

Also, from a comparison of Sample H and Sample J, the effect of theimmersion in an alkaline process liquid, i.e. the corrosion-resistanceeffect of the passive film, can be seen.

FIG. 6 shows data obtained by measuring the corrosion current densitiesof test pieces having had passive films formed on them with theimmersion time in the alkaline process liquid, i.e. aqueous NaOH, in thepassivation treatment step ST02 in the process shown in FIG. 1A madedifferent in each case, the solid line showing cases where air bubblingwas carried out and the dashed line showing cases where air bubbling wasnot carried out. The vertical axis of the graph shows corrosion currentdensity (in units of μA/cm²) and the horizontal axis shows alkalineprocess liquid immersion time (in units of minutes).

When air bubbling is not carried out, 15 min is needed to reach thecorrosion current density 3.6 μA/cm² of the related art case of whentreatment is carried out with nitric acid as the process liquid, butwhen air bubbling was carried out, 3.6 μA/cm² was reached in 10 min.This is the basis for the alkaline process liquid immersion time of 10min of the present invention.

Thus, by performing air bubbling, it is possible to shorten the alkalineprocess liquid immersion time, i.e. the time needed for the passivationtreatment step, and it is possible to raise the productivity ofseparators.

In FIG. 7, in the passivation treatment step ST02 in the process shownin FIG. 1A, the process liquid for performing the passivation treatmentwas altered and for each process liquid the pH was measured at intervalsof a predetermined time while bubbling was carried out. The verticalaxis of the graph shows process liquid pH and the horizontal axis showsbubbling time (in units of min). The compositions of the process liquidsare as follows.

Process Liquids

-   -   Process Liquid L: 1.1 wt % aqueous Na₂CO₃    -   Process Liquid M: 0.00002 wt % aqueous NaOH with 0.55 wt %        aqueous Na₂CO₃ added    -   Process Liquid N: 0.0003 wt % aqueous NaOH with 0.11 wt %        aqueous Na₂CO₃ added    -   Process Liquid P: 0.003 wt % aqueous NaOH with 0.011 wt % Na₂CO₃        added    -   Process Liquid Q: 0.0005 wt % aqueous NaOH

In Process Liquid Q, because it is aqueous NaOH only, the pH of processliquid gradually falls a long way down as bubbling continues.

In Process Liquids M, N and P, as bubbling continues, the higher is theconcentration of aqueous Na₂CO₃ the smaller is the degree of the fall inthe pH of the process liquid, and in Process Liquid M it is almost flat.

In Process Liquid L, because the concentration of aqueous Na₂CO₃ ishigher than in the other process liquids, the degree of the fall in pHis the smallest, that is, the pH buffer effect in Process Liquid L isthe greatest.

Thus, when air bubbling is carried out, if an alkaline solution having apH buffer added or a pH buffer effect is used, pH fall can be suppressedand it is possible to make the quality of the passive film stable.

Also, to confirm the influence on corrosion resistance of the aqueousNa₂CO₃ which is the pH buffer, the following corrosion test was carriedout. TABLE 2 CORROSION NaOH Na₂CO₃ CURRENT DENSITY PROCESS LIQUID wt %wt % pH μA/cm² S 0.0004 0 10.84 3.6 T 0 1.1 11.01 3.6 U 0.0003 0.1110.81 3.5 V 0.0003 0.33 10.92 3.6 W 0.00002 0.53 10.99 3.7

In the corrosion test, the process liquid was varied in the passivationtreatment step ST02 in the process of FIG. 1A, passive films were formedon test pieces with the respective process liquids, and the corrosioncurrent densities of the test pieces were measured.

The process liquid temperature at the time of passivation treatment was50° C., the immersion time was 10 min, the amount of oxygen dissolved inthe process liquid by air bubbling was 7.0 to 7.9 mg/1000 cm³, and afterthe passivation treatment drying by heating was carried out at 110° C.for 10 min. The corrosion test conditions were the same as in the caseof FIG. 2.

Process Liquid S was 0.0004 wt % aqueous NaOH with a pH of 10.84. Theresult was that the corrosion current density was 3.6 μA/cm².

Process Liquid T was 1.1 wt % aqueous Na₂CO₃ with a pH of 11.01. Theresult was that the corrosion current density was 3.6 μA/cm².

Process Liquid U was 0.0003 wt % aqueous NaOH with 0.11 wt % aqueousNa₂CO₃ added, and had a pH of 10.81. The result was that the corrosioncurrent density was 3.5 μA/cm².

Process Liquid V was 0.0003 wt % aqueous NaOH with 0.33 wt % aqueousNa₂CO₃ added, and had a pH of 10.92. The result was that the corrosioncurrent density was 3.6 μA/cm².

The composition of Process Liquid W was 0.00002 wt % aqueous NaOH with0.53 wt % aqueous Na₂CO₃ added, with a pH of 10.99. The result was thatthe corrosion current density was 3.7 μA/cm².

Thus, adding aqueous Na₂CO₃, which is a pH buffer, to the alkalinesolution, or using a pH buffer Na₂CO₃ solution showing alkalinity, doesnot influence corrosion resistance.

The alkaline solution of this invention is not limited to aqueous NaOH,and may alternatively be an aqueous solution of Na₂CO₃ (sodiumcarbonate), NaH₂PO₄ (sodium dihydrogen-phosphate), Na₂HPO₄ (disodiumhydrogen-phosphate), Na₃PO₄ (trisodium phosphate), Na₄P₂O₇ (sodiumpyrophosphate), Na₂O.nSiO (water glass (sodium silicate), Na₂B₄O₇(sodium tetraborate), KOH (potassium hydroxide), K₂CO₃ (potassiumcarbonate), KH₂PO₄ (potassium dihydrogen phosphate), K₂HPO₄ (dipotassiumhydrogen-phosphate), K₃PO₄ (tripotassium phosphate), K₄P₂O₇ (potassiumpyrophosphate), or K₂B₄O₇ (potassium tetraborate).

The pH buffer of the invention is not limited to Na₂CO₃, and mayalternatively be borax (Na₂B₄O₇), amino acid, alanine, aspartic acid,cystine, glutamine, glycine, isoleucine, leucine, methionine,phenylalanine, or proline.

Also, in this invention, although air bubbling was carried out forhydroxide formation promotion, there is no limitation to this, andalternatively showering (pouring the alkaline solution into the processtank in the form of a shower) may be carried out to dissolve more oxygenin the alkaline solution.

With reference to FIG. 8A a separator manufacturing method will bedescribed.

(A) Stainless steel thin sheets 10 are press-formed in a press mold 31(specifically, this is an upper die 32 and a lower die 33), to makemultiple press-formed starting members 11.

(B) The press-formed starting members 11 are cleaned. Specifically,because grease applied to make good the lubrication between thestainless steel thin sheet 10 and the press mold 31 when the stainlesssteel thin sheet 10 is press-formed is adhered to the press-formedstarting member 11, a cleaning liquid (the details of which will bediscussed later) is sprayed on the press-formed starting member 11 withspraying devices 36 to remove the grease (degreasing), and then water(the details of which will be discussed later) is sprayed on thepress-formed starting member 11 with other spraying devices to remove(rinse off) the cleaning liquid adhered to the press-formed startingmember 11.

(C) An alkaline solution for passivation treatment (hereinafter, simply“passivation treatment liquid”) is sprayed onto the press-formedstarting member 11 with spraying devices 38 to effect passivationtreatment, and a passive film is formed on the press-formed startingmember 11.

At this time, because the spraying causes oxygen to dissolve in thepassivation treatment liquid and the amount of oxygen dissolved in thepassivation treatment liquid increases, hydroxide formation is promoted.For example, when a passive film having Fe, Cr, Ni hydroxides and oxidecomponents is formed, when the separator is assembled to a fuel cell,even if the atmospheres around the electrodes become acidic duringelectricity generation of the fuel cell, acid corrosion can besuppressed, and stable generation can be maintained over a long period.The mechanism of the promotion of hydroxide formation is as describedabove.

(D) Because passivation treatment liquid has adhered to the film-formedmember 17 obtained by forming the passive film, water is sprayed ontothe film-formed member 17 with other spraying devices to rinse thefilm-formed member 17.

(E) The film-formed member 17 is dried by heating in a thermal dryer 18.When the film-formed member 17 is dried, the separator is finished.

In FIG. 8B, the press-formed starting member 11 consists of a basematerial 22 and a conductor 24 (for example Cr₂B) included in this basematerial 22, and has the conductor 24 partially exposed at the surfaceof the base material 22.

FIG. 8C shows a passive film 26 formed at the surface of the basematerial 22 of the film-formed member 17. The conductor 24 projects fromthe passive film 26 (i.e. is in a partially exposed state).

In FIG. 9, the processes from degreasing to drying by heating ofstainless steel members of an embodiment (this embodiment) and acomparison example are compared. For the embodiment, the content of FIG.1 will be explained in detail. The upper half shows the embodiment andthe lower half shows the comparison example. STXXX in the figure denotesa step number.

First, the embodiment will be described step by step.

ST11: Spray degreasing is carried out. That is, degreasing is carriedout by an alkaline solution for cleaning at 60° C. serving as theabove-mentioned cleaning liquid being sprayed at the press-formedstarting member at a pressure of 1 kgf/cm². The time required for thisis 1 minute, and the amount of liquid used is 10 L (liters) (the amountsprayed per unit time is 10 L/min; this is the same in the embodimentsbelow).

The alkaline solution for cleaning is a detergent (trade name: PakunaSpray 50-N, maker name: Yuken Industries Co., Ltd.) made by adding asurfactant (polyoxyethylene=alkyl ether C12-15) to carbonate, phosphate,carboxylate alkaline salts (mainly sodium salt).

ST12: A spray rinsing W1 is carried out. That is, to remove alkalinesolution for cleaning adhered to the press-formed starting member,rinsing is carried out by mains water or industrial water being sprayedonto the press-formed starting member as washing water. The timerequired for this is 0.25 minutes, and the amount of liquid used is 2.5L.

ST13: A spray rinsing W2 is carried out. That is, to remove mains wateror industrial water adhered to the press-formed starting member, rinsingis carried out by spraying ion-exchange water onto the press-formedstarting member. The time required for this is 0.25 minutes, and theamount of liquid used is 2.5 L. The spray rinsing W1 and the sprayrinsing W2 described above are provided to reduce costs by using mainswater or industrial water, which are cheaper than ion-exchange water,and then remove any chlorine component included in the mains water orindustrial water by using ion-exchange water. A chlorine component wouldimpede the passivation reaction.

ST14: A spray passivation treatment is carried out.

That is, aqueous NaOH (pH 9 to 12, 40 to 60° C.) is sprayed onto thepress-formed starting member as a passivation treatment liquid to effecta passivation treatment. The time required for this is 10 minutes.

ST15: A spray rinsing W3 is carried out. That is, to remove passivationtreatment liquid adhered to the film-formed member, the film-formedmember is rinsed by being sprayed with ion-exchange water. The timerequired for this is 0.5 minutes, and the amount of liquid used is 5 L.

ST16: Drying by heating of the film-formed member is carried out at 100to 200° C. The time required for this is 10 minutes.

From the above, the total time required in the embodiment fromdegreasing to thermal drying is 22 minutes. And, the total amount ofliquid used in the spray degreasing and spray rinsing W1 through sprayrinsing W3 is 20 L.

Next, the comparison example will be described step by step.

ST101: Ultrasonic degreasing is carried out. That is, ultrasonicdegreasing of the press-formed starting member is carried out using analkaline solution for cleaning. The time required for this is 5 minutes,and the amount of liquid used is 150 L (the immersion method is theoverflow method, and the amount of overflow per unit time is 30 L/min;this is the same in the comparison examples below).

ST102: Immersion degreasing is carried out. That is, the press-formedstarting member is degreased by being dipped in an alkaline solution forcleaning. The time required for this is 5 minutes, and the amount ofliquid used is 150 L.

ST103: An immersion rinsing W1 is carried out. That is, the press-formedstarting member is rinsed by being dipped in mains water. The timerequired for this is 1 minute, and the amount of liquid used is 30 L.

ST104: An immersion rinsing W2 is carried out. That is, the press-formedstarting member is rinsed by being dipped in ion-exchange water. Thetime required for this is 1 minute, and the amount of liquid used is 30L.

ST105: An immersion passivation treatment is carried out. That is, thepress-formed starting member is dipped in an alkaline solution forpassivation treatment, and a passivation treatment is thereby effected.The time required for this is 10 minutes.

ST106: An immersion rinsing W3 is carried out. That is, the film-formedmember is rinsed by being dipped in ion-exchange water. The timerequired for this is 6 minutes, and the amount of liquid used is 180 L.

ST107: The film-formed member is dried by heating. The time required forthis is 10 minutes.

From the above, the total time required in the comparison example fromdegreasing to thermal drying is 38 minutes. And, the total amount ofliquid used in the degreasing and immersion rinsing W1 through immersionrinsing W3 is 540 L.

From the embodiment and the comparison example described above, thespray method of the embodiment made it possible to shorten the requiredtime by 16 minutes and reduce the amount of liquid used by 520 L.

In FIG. 10A and FIG. 10B, an alkaline solution for cleaning was used toperform a comparison of the cleaning powers (degreasing powers) of spraycleaning and ultrasonic and immersion cleaning.

With reference to FIG. 10A, a flow of sample preparation and effectconfirmation will be described step by step.

ST21: Etching is carried out to partially expose conductors on astainless steel test piece.

ST22: Oil (a mixture of grease, machine oil and corrosion inhibitor) isapplied to the sample piece.

The above-mentioned grease is trade name: COSMO GREASEMAX #1,composition: lubricating oil base oil about 91 wt %, thickening agent(lithium soap) about 7 wt %, lubricating oil additive about 2 wt %,maker name: Cosmo Oil Lubricants Co., Ltd.

The machine oil is trade name: No. 630 Press Machining Oil, composition:petroleum hydrocarbons (mineral oil) about 50 wt %, chlorine extremepressure additive 10 to 50 wt %, sulfur extreme pressure additive 1 to10 wt %, maker name: Nihon Kohsakuyu Co., Ltd.

The corrosion inhibitor is trade name: Non-Ruster P30F, composition:rust inhibitor additive, film-forming agent, solvent, maker name:Yushiro Chemical Industry Co., Ltd.

ST23: The test piece is cleaned with the alkaline solution for cleaningmentioned above.

At this time, (1) in the case of spray cleaning, the solutiontemperature is 60° C., the cleaning time is 1 minute, and the spraypressure is 1 kgf/cm², and (2) in the cases of ultrasonic cleaning andimmersion cleaning, the solution temperature in the ultrasonic cleaningis 40° C. and the cleaning time is 5 minutes, and the solutiontemperature in the immersion cleaning is 40° C. and the cleaning time is5 minutes.

ST24: The test piece is rinsed.

ST25: The test piece is dried.

ST26: To confirm the effect of the cleaning, the test piece is dipped inn-hexane solvent to extract grease remaining on the test piece bydissolving it in the n-hexane solvent.

ST27: The n-hexane solvent is spectroscopically analyzed with aninfrared spectroscope, and its oil content is measured.

FIG. 10B is a graph showing the oil content of the test pieces obtainedin FIG. 10A, the vertical axis showing oil content remaining on the testpiece (in units of mg/cm²) and the horizontal axis showing the cleaningmethods.

In the case of no cleaning, the oil content was 3.5 mg/cm², in the casesof ultrasonic cleaning and immersion cleaning it was 0.55 mg/cm², and inthe case of spray cleaning it was 0.15 mg/cm², so that with spraycleaning the oil content was 73% lower with respect to ultrasoniccleaning and immersion cleaning. That is, the cleaning power (degreasingpower) of spray cleaning is highest.

In FIG. 11A and FIG. 11B, using an alkaline solution for cleaning, acomparison of the variation with cleaning time of the cleaning powers(degreasing powers) of spray cleaning and immersion cleaning was carriedout.

With reference to FIG. 11A, a flow of sample preparation and effectconfirmation will be described step by step.

ST31: Etching is carried out to partially expose conductors on astainless steel test piece.

ST32: An oil (the same one as that used in FIG. 10) is applied to thetest piece.

ST33: The test piece is cleaned (degreased) with the alkaline solutionfor cleaning mentioned above.

At this time, (1) in the case of spray cleaning, the solutiontemperature is 40° C., and the spray pressure is 1 kgf/cm². (2) In thecase of immersion cleaning, the solution temperature is 40° C.

ST34: The test piece is rinsed.

ST35: The test piece is dried.

ST36: To confirm the effect of the cleaning, the test piece is dipped inn-hexane solvent to extract grease remaining on the test piece bydissolving it in the n-hexane solvent.

ST37: The n-hexane solvent is spectroscopically analyzed with aninfrared spectroscope, and the amount of oil remaining on the test pieceis measured.

In the experiment, for each cleaning time, test pieces cleaned by spraycleaning and immersion cleaning were respectively prepared by theprocesses described above, and the amounts of oil on the test pieceswere measured.

The cleaning times were made 0 (zero) minutes (no cleaning), 1 minute, 3minutes, 5 minutes, 10 minutes, 15 minutes and 20 minutes.

FIG. 11B is a graph showing the relationship between the amounts of oilon the test pieces obtained in FIG. 11A and cleaning time, the verticalaxis showing amount of oil (in units of mg/cm²) and the horizontal axisshowing cleaning time (in units of minutes).

With spray cleaning, from the oil content of 3.5 mg/cm² of beforecleaning it decreased steeply to 0.14 mg/cm² in a cleaning time of 1minute, and thereafter was roughly flat.

With respect to this, with immersion cleaning, the oil content of 3.5mg/cm² before cleaning decreased in a cleaning time of 1 minute to 1.0mg/cm², and thereafter the oil content fell with time and after 20minutes became approximately the same as in the case of spray cleaning(spray cleaning 0.08 mg/cm², immersion cleaning 0.13 mg/cm²).

Thus, with spray cleaning the cleaning time (degreasing time) can begreatly shortened compared to immersion cleaning.

In FIG. 12, a comparison of variation of cleaning power with rinsingtime with spray rinsing and immersion rinsing was carried out.

With reference to FIG. 12A, a flow of sample preparation and effectconfirmation will be described step by step.

ST41: A test piece made of stainless steel with added boron is prepared.

ST42: The test piece is dipped in an alkaline solution for cleaning for3 minutes.

ST43: In the case of spray rinsing, the amount of water sprayed on thetest piece is made 10 L/minute.

ST44: In the case of immersion rinsing, the amount of water in overflowimmersion is made 10 L/min.

ST45: The test piece is immersed in ion-exchange water for 60 minutes.

ST46: The pH of the ion-exchange water in which the test piece wasdipped is measured.

In the experiment, for each rinsing time, a test piece rinsed by sprayrinsing and a test piece rinsed by immersion rinsing were prepared bythe process described above, and after each test piece was immersed inthe ion-exchange water its pH was measured.

The rinsing times were made 15 minutes, 30 minutes and 45 minutes (sprayrinsing only), 60 minutes and 120 minutes.

FIG. 12B is a graph showing the relationship between the pH of theion-exchange water and the rinsing time, the vertical axis showing pHand the horizontal axis showing rinsing time (in units of sec).

With spray rinsing, at rinsing time 15 minutes the pH had fallen to7.17, and with respect to further increase of the rinsing time the pHremained roughly flat.

With respect to this, in the case of immersion rinsing, at rinsing time15 minutes the pH had only fallen as far as 7.70, and thereafter,although it gradually fell as the rinsing time increased, even atrinsing time 120 minutes the pH had not fallen as far as the pH in thecase of spray rinsing.

Thus, with spray rinsing it is possible to lower the pH in a shortertime compared to immersion rinsing, i.e. quicker rinsing is possible.

In FIG. 13, the corrosion current densities of test pieces (havingfinally undergone drying by heating) having had passive films formed onthem with different process times respectively by passivation treatmentby spraying and passivation treatment by immersion were measured andcompared. Three test pieces were prepared for each set of processconditions to measure the corrosion current densities, and the averagesof the threes were plotted on the graph. The vertical axis of the graphshows corrosion current density (in units of μA/cm²), and the horizontalaxis shows process time of the passivation treatment (in units of min).The dashed line is the necessary value of the corrosion current density(5.1 μA/cm²).

The corrosion test conditions are shown below.

Corrosion Test Conditions

-   -   test solution: sulfuric acid (pH 3, concentration 0.005%,        temperature 90° C.)    -   test piece potential: constant 638.8 mV (set with reference to a        saturated calomel electrode (SCE)), hereinafter referred to as        “638.8 mV vs. SCE”    -   test method: measure corrosion current density after holding the        above test piece potential for 30 minutes

The passivation treatment conditions are shown below.

Spray Method

-   -   process liquid: aqueous NaOH (pH 10.7 to 11, temperature 60° C.)    -   spray quantity: 100 L/minute    -   spray time: 10 minutes    -   thermal drying: 110° C., 10 minutes

Immersion Method

-   -   process liquid: aqueous NaOH (pH 10.8, temperature 50° C.)    -   immersion time: 10 minutes    -   thermal drying: 110° C., 10 minutes

The corrosion current densities in the cases of the spray method and theimmersion method had similar values at each of the process times. Thatis, in the cases of both the spray method and the immersion method, at 3minutes and over the necessary value was achieved, and there was nodifference in the process time between the spray method and theimmersion method.

In FIG. 14 the through resistances (contact resistances) of a separatormanufactured by the spray method and a separator manufactured by theimmersion method have been measured and compared. The vertical axis ofthe graph shows through resistance (in units of M Ω·cm²) and thehorizontal axis shows two conditions of the time of measurement of thethrough resistance.

As the conditions of the time of measurement of the through resistance,in one case two separators were brought into contact with each other andthe through resistance across them was measured, and in the other caseone sheet of carbon paper constituting an electrode was sandwichedbetween two separators and the through resistance across the separators(here written across separators/electrode) was measured. The facepressure across the separators at the time of the through resistancemeasurement was that at which the through resistance stabilized as theface pressure was gradually increased.

The passivation treatment conditions are shown below.

Spray Method

-   -   process liquid: aqueous NaOH (pH 10.7 to 11, temperature 60° C.)    -   spray quantity: 100 L/minute    -   spray time: 10 minutes    -   thermal drying: 110° C., 10 minutes

Immersion Method

-   -   process liquid: aqueous NaOH (pH 10.8, temperature 50° C.)    -   immersion time: 10 minutes    -   thermal drying: 110° C., 10 minutes

The through resistance across the separators and the through resistanceacross the separators/electrode in the cases of the spray method and theimmersion method were equal, and the through resistance across theseparators/electrode was below the target value of 20.5 m Ω·cm².

In this invention hydroxide formation was promoted by the alkalinesolution for passivation treatment being showered, but in addition tothe showering of the alkaline solution for passivation treatment,hydroxide formation may be further promoted by air being blown (i.e.air-bubbled) into the tank in which the alkaline solution forpassivation treatment is stored.

INDUSTRIAL APPLICABILITY

In this invention, because a passive film is formed at the surface of astainless steel member by the stainless steel member being immersed inan alkaline solution of pH 9 to 12 at 40 to 60° C., there is no solvingout of metal ions from the stainless steel member and the passivationtreatment can be carried out by immersion in a single solution, and costreductions including waste processing can be achieved. And at the sametime, the process can be made rapid by cleaning such as degreasing andthe passivation treatment being carried out by spraying. Thus, theinvention is useful in the manufacture of fuel cells.

1. A stainless steel member passivation treatment method for forming apassive film on a surface of the stainless steel member, said methodcomprising the steps of: immersing the stainless steel member in analkaline solution of pH 9 to 12 at 40 to 60° C.; and air bubbling thealkaline solution, added with a pH buffer, or provided with a pH bufferaction, whereby the amount of oxygen dissolved in the alkaline solutionis increased to promote the formation of hydroxides constituting thepassive film, and by carbon dioxides dissolving in the alkaline solutionfalling of its pH is suppressed.
 2. (canceled)
 3. A stainless steelmember passivation treatment method according to claim 1, characterizedin that a stainless steel member having finished the immersion step isdried by being held at 100 to 200° C.
 4. A stainless steel memberpassivation treatment method according to claim 3, characterized in thatthe stainless steel member is a separator for use in a fuel cell.
 5. Amethod for manufacturing a stainless steel separator for use in a fuelcell, characterized in that it comprises: a step of applying a lubricantto a stainless steel thin sheet and press-forming gas flow passages andcooling water flow passages in it; a step of removing lubricant adheredto the stainless steel thin sheet by spraying the press-formed stainlesssteel thin sheet with an alkaline solution for cleaning; a step ofremoving alkaline solution for cleaning adhered to the stainless steelthin sheet by spraying washing water onto the stainless steel thinsheet; a step of removing washing water remaining on the stainless steelthin sheet by spraying ion-exchange water onto the stainless steel thinsheet; a step of spraying an alkaline solution for passivation treatmentonto the stainless steel thin sheet to passivation-treat the stainlesssteel thin sheet; a step of removing alkaline solution for passivationtreatment adhered to the stainless steel thin sheet by sprayingion-exchange water onto the stainless steel thin sheet; and a step ofthermally drying the stainless steel thin sheet.
 6. A method formanufacturing a stainless steel separator for use in a fuel cellaccording to claim 5, characterized in that the alkaline solution forpassivation treatment is a solution of pH 9 to 12 brought to 40 to 60°C.
 7. A method for manufacturing a stainless steel separator for use ina fuel cell according to claim 6, characterized in that the alkalinesolution for passivation treatment is a solution with a pH buffer added.8. A method for manufacturing a stainless steel separator for use in afuel cell according to claim 5, characterized in that the thermal dryingprocess is carried out at 100 to 200° C.
 9. A method for manufacturing astainless steel separator for use in a fuel cell according to claim 5,characterized in that the alkaline solution for cleaning is a solutionmade by adding a surfactant to a basic salt.